Silicon for use in solar cells can be prepared by a chemical vapor deposition (CVD) process, such as the decomposing silane or the direct reaction of silicon tetrachloride with hydrogen or metal vapors at elevated temperatures (CVD). Silicon produced by these processes, however, is polycrystalline with a random grain orientation so that many of the grain boundaries are perpendicular to the incident light on the solar cell, which results in the entrapment of charge carriers at grain boundaries. In addition, chemical vapor deposition produces a deposit of non-uniform thickness because the silicon deposition rate varies with the local temperature and the reactant concentrations. A further difficulty with chemical vapor deposition is that the silicon is deposited not only on the substrate, but also on the reactor walls which wastes material and leads to high processing cost.
Some of these difficulties have been overcome by depositing the silicon on molten metals, one of which contains dissolved zinc (see U.S. Pat. No. 4,225,367) which reacts with the silicon tetrachloride gas above the molten metal to produce zinc chloride, which is vaporized, and silicon, which is deposited on the molten metal. This process prevents the deposition of the silicon on the walls of the reactor, provides a more uniform thickness of silicon, and results in columnar growth of the silicon so that grain boundaries are perpendicular to the surface and to the incident light.
While this process is an improvement over the chemical vapor deposition process, it is limited to the production of flat sheets of silicon. Also, it presents a control problem in that the zinc in the molten metal becomes depleted and must be replaced, which makes it difficult to maintain a uniform zinc concentration.